(Not Applicable)
(Not Applicable)
The present invention relates generally to measuring devices and more particularly to an inductor-capacitor (LC) probe used for measuring surface conductivity.
Highly conductive coatings such as Indium Tin Oxide (ITO) are applied to glass/polycarbonate windshields and plastic lamp covers of stealth vehicles for scattering treatment of radar signals. For graphite and fiberglass composite surfaces, silver or similar conductive surfaces are extensively used. The electrical properties of these conductive coatings are characterized by surface resistance in ohms/square (xcexa9/sq).
A four-point probe is a direct current (DC) resistance measurement device that requires direct contact with the conductive surface. When fabrication of these components is complete, the windshield ITO coating is protected with a urethane topcoat. The silver painted composite surface is painted with a urethane-based color paint. These topcoats insulate the conductive coating and inhibit electrical testing of the conductives using a four-point probe.
Silver paint has a service life of several years. Over time silver paint loses conductivity, e.g., due to oxidation. Routine and repair inspections of the coating""s conductivity are required. Thus, there is a need for a way to measure a conductive layer""s surface resistance through an insulating topcoat.
Surface gaps and seams are a major scattering source of radar signals. To suppress scattering of radar signals, surface gaps are typically filled with a conductive caulk. Most conductive caulks are made up of polymers and metal particles. Nickel particles are commonly used due to the low cost and chemical inertness. Shrinkage of the cured polymers consolidates the metal particles and brings DC conductivity upon the caulk. Since conductive caulk is installed in surface gaps, the caulk is subjected to ambient and mechanical agitation. Conductivity of the caulk degrades as the polymers age and lose their elasticity. The insulating color paint level prevents DC testing of the aging caulk. Inspection of the conductive caulk is further complicated by the nickel particles"" magnetic properties. Neither eddy current nor magnetic induction is effective for moderately conductive, magnetic materials because eddy current in the conductor counteracts the magnetic induction produced by the metal""s ferromagnetism. Thus, there is a need for a way to measure the conductivity of moderately conductive, ferromagnetic materials.
A non-contact surface conductivity measurement probe for determining the conductivity of a material is disclosed. The probe includes an oscillator, a sensor made up of an LC circuit, a detector for detecting the response of the LC sensor circuit and a processor, such as a microprocessor, for converting the detected signal to surface conductivity. The detector may be a frequency counter that detects changes in, the resonant frequency of the LC circuit or the detector may be an RF level detector that detects changes in the signal magnitude across the LC circuit. The LC circuit includes a capacitor (c) and a sensor coil (L). Inductance and dissipation factor of the sensor coil are varied depending on the conductivity and permeability of the material near the sensor coil.
Preferably, the oscillator and the LC sensor circuit are combined to form a free running oscillator. More preferably, the oscillator is a Colpitts oscillator circuit.
Preferably, the oscillator frequency is about 21 MHz.
In a typical application, the sensor coil is maintained at a fixed distance from the test surface. Surface resistance may be measured by a shift in the resonant frequency or a change in the oscillatory output level of the LC circuit. The shift in the resonant frequency or the signal magnitude is correlated to a set of known thin film resistance standards to yield the surface resistance of the test surface. The sensor""s response to the thin film resistance standards may be stored in the processor circuit. In exemplary embodiments, the probe has a surface resistance range of about 0.01xcexa9/sq to about 30xcexa9/sq.
The test surface or target surface may be a non-magnetic conductive material. Measurement of the resonant frequency shift is the preferred detection method. If the material is a non-magnetic conductive material, a higher surface conductivity induces a higher eddy current on the conductor surface and a greater magnetic field is created. This magnetic field counteracts the sensor""s driving field. Larger counteracting or opposing magnetic field results in larger resonant frequency shift (increase).
The test surface may be a ferromagnetic material. Measurement of the oscillator output level is the preferred detection method. If the material is a ferromagnetic material, a higher surface conductivity couples a heavier load to the LC circuit and results in a lower oscillator output level.
The probe may include a display device for displaying the conductivity measurement.
The test surface may comprise a Magnetic Radar Absorbing Material (MagRAM). Measurement of the resonant frequency shift is used to determine the thickness of the MagRAM coating. Thicker MagRAM coating increases the magnetic flux linkage and causes the sensor coil""s inductance to increase. As a result, the resonant frequency decreases with larger MagRAM coating thickness. MagRAM may have a conductive or a non-conductive substrate. The MagRAM coating may be covered by a non-conductive paint coat.